Hydrostratigraphy and hydrogeophysical studies to delineate fresh and saline aquifer boundaries in Lesser Cholistan of Pakistan

The differentiation of saline water and fresh water interfaces is a key objective in ground water exploration and management. Bahawalpur is the twelfth biggest metropolitan area of Pakistan situated in south Punjab near the bank of River Sutlej and lies at 29°59’55” N latitude and 73°15’12” E longitude at an elevation of 521 ft AMSL in the Cholistan area close to the Thar abandon. The study area comprised of Lesser Cholistan experiencing acute shortage of water for inhabitants and livestock as well. The occurrence of fresh water is also challenging because of high salinity in groundwater. The present study is intended to identify hotspots of fresh groundwater zones. To achieve the goal, vertical electrical resistivity and borehole data are used to mark fresh and saline interfaces in groundwater. To achieve the results 230 vertical electric sounding were performed in the study area. A total of 3 to 5 geo-electric layers are identified with modeling along with the processing and interpretation of resistivity data. In the study area, resistivity values are classified as very high (>230 Ω-m), high (230–100 Ω-m), medium (100–40 Ω-m), low (40–20 Ω-m) and very low (<20 Ω-m). Borehole data is used to interpret subsurface lithologies and to calibrate the modeled resistivity curves. The electric resistivity data indicates that thick layers of Quaternary sediments is present in the subsurface that is primarily composed of clay, silt, sand, gravels and some kanker. Inversion technique is applied to generate 2D subsurface resistivity maps to delineate fresh and saline water zones. The generated 2D resistivity maps at variable depth above and below water table and formation resistivity maps are successfully utilized to differentiate fresh and saline water zones. The identification of a saline water aquifer within sediments rich in clay was made possible by the observation of very low resistivity measurements in the southern region. Conversely, the detection of relatively high resistivity values, coupled with the presence of sand and gravel deposits in the northern section of the lesser Cholistan area, provided compelling evidence of the existence of fresh groundwater. These findings have significant implications for the management of water resources in the region, as they provide valuable insights into the distribution and availability of groundwater resources for future use.

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Introduction
The sustainable supply of freshwater is one of the key parameters in urbanization and the socio-economic development of an area (Muhammad et al., 2022).Surface water, which includes streams, canals, lakes, rivers, etc. and groundwater are two primary cheap and easily available sources of freshwater (Hasan, 2017).Due to high solubility of different solutes water is capable to dissolve suspend, soak and adsorb various impurities such as arsenic, sulfides, and chlorides, organic and inorganic etc. (Kosemani and Oyelami, 2017).Pakistan is among those countries that are facing freshwater scarcity.The whole country is situated in arid to the semi-arid regions with low precipitation.The majority of the population relies on groundwater for their domestic, agricultural and industrial needs.The over-abstraction of groundwater is one of the main reason in enhancing salinity in the groundwater whereas the poor dumping strategies of solid waste is the main source of groundwater contamination (Rasool, 2017).
Cholistan deserta part of Thar desertcovers an area of 25,800 km 2 is the largest desert in the southern Punjab province of Pakistan (Figure 1).This desert is divided into two part: Lesser Cholistan and Higher Cholistan.Bahawalpur city is the largest city in the Lesser Cholistan while the Higher Cholistan is uninhabited with high sand dunes upto 100 m.Around 16,000 km 2 of the district is comprised of the Cholistan Desert that is covered by dunes of low height (Farooq et al., 2001).The density of dunes increases towards the northwest.The economy of this district mainly based on agriculture thus the continuous supply of freshwater is the backbone in its economy.
However, the existing groundwater is highly contaminated by hazardous pollutants.The analysis of groundwater performed by PCRWR from 2002 to 2015 demonstrates that the groundwater on most of the localities in this district is not safe for drinking.In the southern part of the Bahawalpur area, between Fort Abbas and Derawar Fort, there is a zone of reduced salinity.This relatively low-lying area corresponds to the ancient channel of Ghaggar (Hakra) River.Although the Ghagger (Hakra) channel is now generally dry, floodwaters occasionally flow through it from the east, near Fort Abbas, in periods of uncommonly high rainfall.Available data indicate that the upper part of the aquifer along this channel contains water of about 4,000 ppm TDS.A deep-water sample (~ 410 m depth) collected near to Derawar Fort contained high salt concentration up to 25,400 ppm (Geyh and Ploethner, 2008).The study area is a part of the Punjab Platform that is a subdivision of the Upper Indus Basin.The study area consists of alluvial Aeolian plains (Farooq et al., 2001).The area along the Sutlej River comprised the alluvial plain whereas the rolling dunes cover the Aeolian plain of the Cholistan Desert.The planning and management for better quality water supply are not much efficient in Bahawalpur City (Anwar and Bureste, 2011).The use of hydrogeological investigations with the help of some appropriate geophysical techniques and methods to assess the groundwater potential in confined or unconfined aquifer system in a particular area is very common (Courteaud et al., 1997;Zouhri et al., 2004;Schrott and Sass, 2008;Edmund, 2009;Okoro et al., 2010;Riddell et al., 2010;Muchingami et al., 2012;Utom et al., 2012;Vouillamoz et al., 2012;Farid et al., 2013;Mondal et al., 2013;Farid et al., 2014).The spatial and vertical distribution of different characteristics of aquifer such as facies distribution and hydraulic parameters variations are fundamental for the study of alluvial deposits.
Since the saline water has an enormous amount of dissolved solids (TDS), therefore, modeling and delineation of fresh and saline water boundaries in the subsurface are necessary before planning of water abstraction and installation of tube wells.The primary objective of this work is to delineate fresh and saline water interfaces and to map the fresh water aquifer boundaries.

Geology and Geomorphology of the Area
Pakistan is situated at the collisional boundary of the Indian and Eurasian Plates, which creates a significant number of intermontane basins and valley systems (Farid et al., 2018).Indus basin is one of the basins produced under the action of local tectonic activities.The sediments generated by the erosion of the surrounding mountains are transported into this basin under the action of sedimentary transport agents.These sediments may vary in size from very coarse alluvial fan gravels and channel deposits with high hydraulic conductivities to very fine lacustrine clays with low hydraulic conductivity and less permeability.Thus, the grain size of the sediments, lithology and geomorphic nature of the deposited sediments are the important agents for the development of clastic fluvial aquifers and their hydraulic characteristics (Fetter, 1994;Bowling et al., 2005).Since the study area is far away from the source of sediments therefore, fine-grained silt/clay to medium to coarse-grained sand are present only.
The borehole data suggests that a 457 m thick cover of alluvium sediments comprising of fine-grained silt and clay to medium to coarse sand of Late Quaternary age is present throughout the area (Shamsi et al., 1965).This cover is of heterogeneous in nature with significant lateral and horizontal variation in lithology (Kemal et al., 1965;Hassan, 1965).Thus, more than one aquifer system may be present in the study area.According to NESPAK report (1980), the alluvium is saturated up to 1,000 m depth.No continuous layer of clay is reported above the aquifer, thus an unconfined type of aquifer may present in the area.However, locally confined or semi-confined aquifer conditions may prevail.The landforms are developed by the action of wind and river and consist of two units: flood plain deposits and aeolian deposits.The width of flood plains varies from 10 -20 km in different localities.According to geomorphological classification, the study area consists of level plains, channel infills, basin, and levees as shown in Fig. 2. The sediments deposited in the area are mainly fine to medium grains silt, clay, and sands in the association of Kanker.Since the area is far away from the source of sediments, therefore coarser sediments such as gravels are very rare.The sand plains of Cholistan Desert comprised of some medium-height sandy ridges and inter-dunal hollows of Hakra River.These deposits are composed of fine to medium silty sand and silt with low permeability.These sediments are well rounded, well-sorted, and transported by wind from adjacent arid zones (Fig. 3 & Fig. 4).An old channel of Hakra River forms a freshwater channel (Geyh and Ploethner, 2015).In the 16 th century, this river was ceased running (Wilhelmy, 1969) and only rainwater flow in it until mid of the 20 th century.
This former fluvial environment exhibits a flat, hardpan surface on which dunes irregularly rest up to10 m high.The upper layer of the floodplain comprises fine-grained silt to sandy loam and silty clays of low permeability but good porosity.Thus, rainwater may accumulate in depressions and may stay for a couple of days to several weeks.Since 1960, some portion of this desert has been converted into cultivated land after the construction of canals starting from Sutlej River.The area receives less than 200 mm of rainfall annually.Rainwater and the Sutlej River are the main recharge sources in the study area.Due to the presence of less permeable top cover on the surface, rainwater does not penetrate to a deeper extent.
The recharge rate can be divided into deep and shallow categories.The palaeohydrological situation can be used to explain different recharge rates for shallow and deep aquifer layers.No reliable temporal information is available in the western part of the Cholistan area.However, pluvial conditions existed between 13 000 and about 4000 years ago in the eastern part of the Cholistan (Singh et al. 1972(Singh et al. , 1974;;Wasson et al.;1984).According to literature, two pluvial periods may have existed from 13,000 to 8,000 years and 7,000 to 4,000 years (Geyh and Ploethner, 2015). A

Methodology
Electrical resistivity has long been used as a method to detect the subsurface hydrogeological and geomorphologic features with considerable success (Stewart et al. 1983;Courteaud et al. 1997;Louis et al., 2002;Zouhri et al., 2004;Riddell et al., 2010).-1D computer program. 2000;Zananiri et al., 2006;Sultan et al., 2009;Akhter et al., 2012;Farid et al., 2013;Farid et al., 2014;Muhammad and Khalid, 2017), using information derived from lithology logs, ground water levels, geologic maps and EC maps. 2 D models are generated and plotted against the resistivity values verses depth and consist of several horizontal layers separated according to discrete band of resistivity as shown in Fig. 5a-f.In general calibration between lithology and resistivity is established using available borehole information and resistivity data and is shown in Table 1 and in Fig. 6.

Result and discussion
Interpretations are made for three to five numbers of geo-electric subsurface layers in the study area.The resistivity and depth/thickness of each layer is distinctive.Electrical resistivity value of each geo-electric layer which can be implemented to characterize the probable underlying lithology, is a function of particle size, porosity, and fluid type.A correlation between the interpreted underlying lithology and true resistivities is created by simultaneously examining the processed data from 230 VES probes and lithological logs of boreholes (Table 1).Furthermore, the lithological logs of boreholes closest to the respective VES probes are used to calibrate the true resistivities (Fig. 6).
Clay, silt, and sand comprise the alluvial deposits with gravel and pebbles of different depths, can be encountered in boreholes that serve as aquifers.The southern portion of the lesser Cholistan has clay-rich sediments as a result of the intrusion of salt contents, and the findings of the resistivity inversion provide confirmation of this.The interpretation of VES results in this area was challenging due to the relatively low difference of resistivity readings between shallow and deep alluvium.Sand and gravel sediments are more prominent in the north and northwestern part of the study area, where they serve as a good aquifer material for fresh groundwater, and clay rich sediments are more dominant in the south and southeast, where it serves as a saline water aquifer, according to borehole data shown in Fig. 4. According to Fig. 2 digital elevation map of Cholistan, where the elevation decline from northeast to southwest, the streams run in a north to southwest direction.The digital elevation map and the ground water level are interrelated.The ground water level changes across the Lesser Cholistan.In the northeast at well TH-50, the water table is around ≤ 12m deep, while in the southwest borehole TH-91, it is about ≤ 17m deep.As can be seen in Fig. 8a-h, the electrical resistivity survey findings indicate that the south portion of the Lesser Cholistan has significantly lower resistivity values than the north side.In the south region of the study area, where the depth of penetration in the VES findings is very low, resistivity data results confirm that higher proportions of clay-rich or salty sediments prevent the electric current from flowing at greater depths.The dry sand, gravel, and boulder sediments are linked with high resistivity values that are above the water table and close to the surface, whereas the freshwater saturated with sand-gravel, and boulder sediments are linked with high resistivity values that are below the water table.The primary aquifer in the region, which is largely on the north and northwestern side of the area, is mainly composed of water-saturated sand-gravel sediments with resistivity values between 20 and 200 Ω.m.The saline sediments are predominantly found on the research area's southern side, with resistivity values < 10 Ω.m above and below the water table.
The research region is segmented into saline and fresh water aquifers because of the salinity factor.
To study the fluctuation in resistivity values in the lesser Cholistan, a variety of resistivity maps are prepared at various depths, above and below the water table, and these maps are displayed in  In order to comprehend the hydrostratigraphy of the research region, the profile A-A′ is chosen on the base map (Fig. 8).Interpolation of resistivity data along the profile AA' yields the resistivity cross-section AA' (Fig. 9a&b).The electrical resistivity processed data and borehole data closest to the profile A-A′ are used to create 2D cross sections (Fig. 9a) along this profile.The profile's interpreted outcomes are displayed in Fig. 10a

Conclusions
Groundwater salinity can be determined using a VES survey, which can also be used to detect subsurface aquifer layers.In the study area, VES survey was carried out in 230 pre-selected locations.The groundwater aquifer conditions in the lesser Cholistan area of the district of Bahawalpur were evaluated spatially using the VES findings and lithological logs of boreholes.In the research area, three to five geo-electric subsurface layers with varying thicknesses are evaluated as an outcome of processing geophysical data.A number of resistivity maps are generated at different depths, both above and below the water table.The distribution of fresh and saline water aquifers in the study area is determined by comparing all of the electrical resistivity maps.The modeling and re-inversion of 1D electrical resistivity data collected in the lesser Cholistan delineated the boundary between the saline water and freshwater zones, with remarkable achievement.Borehole data from several localities throughout the region were used to map the near surface lithology and to calibrate the modeled resistivity curves.At each resistivity point, the value of the formation resistivity was calculated and validated using the lithologies of drilled boreholes.The created 2D resistivity surface maps enabled in describing the fresh and saline water zones at various depth levels.Very low resistivity measurements in the southern portion indicated the occurrence of a saline water aquifer with clay rich sediments.On the other hand, the comparatively high resistivity values with sand and gravel deposits in the northern segment of the lesser Cholistan strongly indicated the presence of fresh groundwater.
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Fig. 1 :
Fig. 1: Base map of the study area showing VES and Borehole locations.

Fig. 2 :
Fig. 2: Digital elevation model shows major geomorphology of the area.
few numbers of studies related to hydrogeophysical investigations and groundwater quality are available in the literature.In 1968, USGS conducted a study on groundwater hydrology of the Punjab Province.According to this study, Most of the area of district Bahawalpur comprises highly saline water with TDS> 5000 ppm except a narrow freshwater corridor along the Sutlej River and an old channel of Hakra River between Fort Abbas to Derawar Fort.Farooq et al. (2001)      conducted a study on physio-chemical analysis of shallow water samples collected from 20 -70 m depth.They concluded that the groundwater is highly mineralized in most of the area of Rahim Yaar Khan District with a high concentration of TDS (27,000 ppm) and high sodium absorption ration ~ 54 mg/l).

Fig. 4 :
Fig. 4: Boreholes in the study area representing the shallow subsurface geology.
Electrical resistivity in addition to the lithologic makeup of the aquifer depends upon the fluid contents of the aquifer.The methodology involve digitations of existing data and the acquired data set.The acquired data comprise of vertical electrical soundings (VES) up to 300 m depth at sparsely distributed locations for convenience, the VES points were generally collected at distance of 4 to 8 km, mostly along the roads, canals and existing tracks as shown in Fig.1.A terrameter SAS 4000 is used for VES data acquisition.All VES data were acquired by vertical sounding using Schlumberger array configuration.The Schlumberger array consists of four electric conductors known as electrodes.The two end conductors are current electrodes and the inside two conductors are the potential electrodes.The potential electrodes are installed at the centre of the electrode array with a small separation, typically less than one-fifth of the spacing between the current electrodes (AB ≥ 5MN).The current electrodes are increased to a greater separation during the survey while the potential electrodes remain in the same position until the observed voltage becomes too small to measure.The data of around 120 test holes, 20 test wells up to 300 m depth is already available from the Water and Power Development Authority (WAPDA) of Pakistan.More than 230 vertical electrical resistivity sounding (VES) data are acquired in different places of the study area was used for the characterization of various subsurface lithologies.VES data is interpreted by using a combination of curve matching technique and computer iterative modeling to find out number of geo-electric layers in the study area.The analysis of VES curves is difficult task and requires that the measured curve be matched with several model curves and each model curve represent different subsurface resistivity distributions.To select the final model is constrained with the help of VES data, lithologs, local geological data, and borehole data of study area, we map different depositional zones in the Bahawalpur district to mark the alluvial aquifer systems as well as the salinity of the groundwater.For calibration and verification of VES results with borehole data, some of the VES points are acquired near to the existing boreholes/tubewells.All the VES data points were modeled (Fig.5) by using computer software IP2Win (IPI2WIN

Fig . 6 :
Fig .6:Borehole logs and modeled resistivity curves representing the calibration between

Fig.
Fig. 7a-h: Maps depicting the distribution of true resistivity across varying depths in two

Fig. 8 :
Fig. 8: Base map of the study area showing VES points, boreholes, and cross section AA'.

Fig. 10 :
Fig. 10: Visualizing Water Quality Distribution with Enhanced Precision: Multi-layered Maps Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

Table 1 :
Cutoff values of resistivity for different lithologies used in interpretation of VES data.
Fig.5a-f: Apparent resistivity data are marked by small circles.Solid black curve represent the apparent resistivity curve.Red curve is the best fitted curve to apparent resistivity data.Solid blue block line is the modeled resistivity (synthetic resistivity).Horizontal axis is the current electrode spacing (AB/2) in meters and vertical axis is the resistivity in ohm meters.